It is recognized that polybenzimidazoles and particularly aromatic polybenzimidazoles are characterized by a high degree of thermal stability. They may be shaped to form fibers, films and other articles of wide utility which show great resistance to degradation by heat, hydrolytic and oxidizing media.
It has been further recognized that solutions of polybenzimidazoles in an organic solvent suitable for extrusion into shaped articles often cannot be stored much more than one to three days without a "phasing out" phenomenon occurring, i.e. a separation of the solution into two phases containing larger and smaller concentrations of polymer. The exact time of the onset of such phase out is unpredictable. This phased out solution is completely unsuitable for extrusion into shaped articles and interferes with routine storage of the polymer solution in an extrusion plant. While the phased out solution can generally be transformed into a homogeneous solution by means of additional mixing procedures, this results in additional expense and inconvenience.
Commonly assigned U.S. Pat. No. 3,502,606 to Anthony B. Conciatori and Charles L. Smart discloses that this undesirable phase separation can be prevented if one incorporates a minor amount of certain additives in the solution of the polybenzimidazole. It is there contemplated that the additive be lithium chloride, zinc chloride, N-methyl morpholine, triethylamine, or triethanol amine. Heretofore lithium chloride has been the additive of choice because of its effectiveness in solving the phase separation phenomenon.
However, it is well known that extended contact of stainless steel equipment (e.g. of common 300 series stainless steel) with halides (e.g., chlorides) has the tendency to lead to the catastrophic stress cracking of the stainless steel. For instance, it is common that various vessels for forming polymer solutions and pressure lines for the same be formed of such stainless steel. The onset of stress cracking can come without warning and necessitates a time consuming periodic visual inspection of the equipment. Should such stress cracks be observed additional replacement costs must be incurred as well as costly down time for the plant. Alternatively, if such procedures are to be eliminated the equipment heretofore must originally be constructed of more expensive alloys which are not as susceptible to corrosive stress cracking. This approach would lead to a significantly greater capital expenditure for a commercial plant.
It is an object of the present invention to provide a stable solution of a polybenzimidazole suitable for use in the formation of shaped articles which does not rely on the inclusion of a lithium chloride additive as is common in the prior art.
It is an object of the present invention to provide a highly stable solution of a polybenzimidazole suitable for use in the formation of shaped articles wherein the additive employed has no known propensity to promote corrosive stress cracks when contacted with common forms of stainless steel for extended periods of time.
It is an object of the present invention to provide a highly stable solution of a polybenzimidazole which is non-corrosive to common 300 series stainless steel thereby making possible significant capital savings during the construction and operation of a plant in which such solution is handled.
It is another object of the present invention to provide a process for forming shaped polybenzimidazole articles (e.g. fibers, films, etc.) wherein neither phase out of the polymer solution involved nor the potential for corrosive attack of the equipment employed need be considered.
It is a further object of the present invention to provide a process which is capable of forming polybenzimidazole fibers and films of unusually high surface area.
These and other objects, as well as the scope, nature, and utilization of the invention will be apparent to those skilled in the art from the following detailed description and appended claims.